1. Field of the Invention
The present invention relates to a liquid crystal display of the TN-FEM (twisted-nematic field-effect) type having liquid crystal molecules with a 90.degree. twist, in which a polarizer and an analyzer are given particular spectral characteristics by setting their polarization axes at a certain angle .theta. to improve the appearance of displayed images on the TN-FEM liquid crystal display especially when an electric field is not applied to the display.
2. Description of the Prior Art
FIG. 1(a ) is a cross-sectional view of a conventional TN-FEM type liquid crystal cell and FIG. 1(b) is a diagram showing polarization axes and of a polarizer and an analyzer, respectively. A liquid crystal cell is positioned between the polarizer 1 (FIG. 1(a)) and the analyzer 7. The liquid crystal cell comprises two glass substrates 2, 2', two transparent electrodes (ITO electrodes) 3, and two insulating films 4 for orienting liquid crystal molecules. The electrodes 3 and the films 4 are positioned between the inner surfaces of the substrates, and a liquid crystal layer 5 of a twisted-nematic structure is inserted between the insulating films 4. The glass substrates 2 and 2' are sealed around their peripheral edges by a spacer 6. The liquid crystal molecules closer to the glass substrate 2 are oriented in the direction of a vector r.sub.1 (FIG. 1(b)), while the liquid crystal molecules closer to the glass substrate 2' are oriented in the direction of a vector r.sub.2. The polarizer 1 has a direction of polarization expressed by vector P, and the analyzer 7 has a direction of polarization expressed by vector A, these directions of polarization being shown in FIG. 1(b) as viewed from immediately above the liquid crystal cell. .theta. is designated as the angle formed between the polarizer 1 and the analyzer 7, and .beta. as the angle of twist (=90.degree.) of the liquid crystal molecules. In FIG. 1(b), the difference angle .DELTA..theta.=(.theta.-.beta.)/2.
FIG. 2 illustrates the general spectral characteristics of light transmitted through the liquid crystal cell shown in FIG. 1(a) when no electric field is applied. The spectral characteristics of the transmitted light are dependent on the anisotrophy .DELTA.n of the refractive index of the light crystal (.DELTA.n=n.sub.3 -n.sub.o where n.sub.3 =refractive index of extraordinary ray n.sub.o =refractive index of ordinary ray), the thickness d of the liquid crystal layer, the spectral characteristics of the polarizer and the analyser, and the angle .theta. formed between the polarization axes of the polarizer and the analyzer.
Recently, liquid crystal displays have been constructed such that the anisotropy .DELTA.n of refractive index of the liquid crystal and the thickness d of the liquid crystal layer are very small in order to improve the viewing angle of the display. As a result, the optical distance .DELTA.n.multidot.d is so small that it is comparative to the wavelengths of visible light rays, thereby causing a strong interference phenomenon. This causes the spectral characteristics of the light transmitted through the TN-FEM type liquid crystal cell to vary greatly in the visible light range, resulting in a coloration or discoloration of the liquid crystal cell. Consequently, the display quality attained by the liquid crystal display is very impaired. This coloring phenomenon is highly dependent on the thickness d of the liquid crystal cell. Since in a liquid crystal cell having a relatively large display area it is difficult to control a uniform cell thickness, the liquid crystal cell is subjected to varying colorations in the area and has a poor display quality.